The present invention relates to a machine for the stress relief of workpieces in which the workpiece is subjected to vibrations at selected speeds of a vibrator and in which the selection of the speeds of the vibrator is taken from a measurement which reproduces the vibratory behavior of the workpiece upon excitation by the vibrator.
A method of stress relief by vibration is described in detail in Federal Republic of Germany utility model 70 05 792 or U.S. Pat. No. 3,677,831. For the stress relieving of workpieces one customarily operates with vibrator speeds of 1200 to 6000 rpm or even up to 12,000 rpm, which corresponds to exciter frequencies of 20 to 100 Hz and 200 Hz respectively. In performing the method one first determines in a measurement run over this operating range, those speeds or frequencies at which the workpiece tends towards strong vibrations which indicate resonate frequencies. The vibratory behavior is generally determined by an accelerometer which is fastened on the workpiece. For stress relief, the workpiece is then subjected to treatment by the vibrator at the speeds at which the workpiece showed resonant frequencies in the preceding measurement run. In the case of workpieces of complicated structure, there are generally so many peaks or maxima in the acceleration value/speed diagram that it is necessary to make a selection of the speeds of the vibrator for the stress relief treatment, in which case one ordinarily picks out only those speeds having clearly pronounced vibration peaks. Not infrequently, the case arises that some of the clearly pronounced peaks represent only harmonic oscillations of a fundamental frequency, so that in the case of those frequencies, stress relief treatment is unnecessary if one has already operated at the corresponding fundamental frequency. In addition, precisely those frequencies which are essential for the stress relief are frequently so poorly pronounced in the workpiece, which has still not been stress relieved, that they are not selected in the acceleration-value/speed diagram during the search for strongly pronounced peaks. It is known, to be sure, that the internal stresses which lie in the microscopic region are not relieved directly by the operating frequencies of the vibrator, but by the harmonics thereof. However, up to now one relied on a clearly pronounced peak occurring in the measurement run upon excitation of such a harmonic which is also within the operating range of the vibrator. Frequently, however, such peaks remain poorly pronounced and are not detected upon the selection of the strongly pronounced vibration peaks, as a result of which the actual stress relief of the workpiece generally remains far below the optimal level.